Image recording of a specimen is most commonly carried out based on a recording of the intensity of light received from the specimen. However, since the interaction of light with a specimen modifies both the light intensity and phase, image recording can also be carried out based on phase detection.
Holography provides a means by which both phase and intensity information can be determined from a single record called a hologram. To obtain a hologram using conventional techniques, a source beam is first split by a beam splitter into a first part, and a second part that is sent to the specimen. The first part is commonly referred to as a reference wave while light from the second part that is scattered from the specimen is referred to as an object wave. The reference wave and object wave are then arranged to interfere with one another, thus producing a complex interference pattern of spaced fringes. This pattern is called the hologram. The hologram essentially freezes the intricate wavefront of light from the specimen. When the hologram is reconstructed (e.g. by illuminating the hologram with a reconstruction wave), a virtual image of the specimen is obtained.
In digital holographic microscopy, the hologram is recorded using digital recording devices (e.g. a digital camera) as opposed to conventional photographic plates. Numerical reconstruction of the hologram is then carried out to reconstruct the wavefront from the specimen digitally.
Digital holographic microscopy has in recent times been implemented in combination with a microscope objective to provide magnification of a test specimen. This implementation has shown great success in the quantitative study of material and life science applications with sub-nanometer resolution. However, it has been found that the microscope objective introduces a phase curvature to the object wave. Since the phase curvature is not present in the reference wave, interference of the object wave and reference wave will produce a hologram that results in a distorted reconstruction. It is therefore desirable to remove or compensate the phase curvature of the object wave.
One approach to remove the phase curvature is by way of numerical compensation in the reconstruction process. To do this, a numerical phase mask is developed. Examples have been described by T. Colomb et al in ‘Numerical parametric lens for shifting, magnification, and complete aberration compensation in digital holographic microscopy,’ J. Opt. Soc. Am. A 23, 3177 (2006), and ‘Automatic procedure for aberration compensation in digital holographic microscopy and applications to specimen shape compensation,’ Appl. Opt. 45, 851 (2006), and by F. Montfort et al in ‘Purely numerical compensation for microscope objective phase curvature in digital holographic microscopy: influence of digital phase mask position,’ J. Opt. Soc. Am. A 23, 2944 (2006). Another approach to remove the phase curvature involves using a reference hologram recorded by the same setup without the test specimen, as proposed by T. Colomb et al in ‘Total aberrations compensation in digital holographic microscopy with a reference conjugated hologram,’ Opt. Express 14, 4300 (2006). Both approaches are done numerically, which complicates the reconstruction algorithm and makes numerical reconstruction a time-consuming process.
It is also known to introduce physically the same curvature in the reference wave, such as through a Linnik interferometer or a Mach-Zehnder interferometer. In these configurations, the use of the measurement optics in the reference arm duplicates the objective measurement optics in the object arm so that curvature of the object wave is compensated during interference by the same curvature in the reference wave. This, however, requires a precise alignment of all the involved optical elements.